Emerging diseases often originate from host shifts of introduced pests or pathogens. Genetic resistance of the host to such diseases might be limited or absent due to the lack of coevolutionary history. We review six examples of major disease outbreaks on native tree species caused by different introduced pests and pathogens that led to large ecological and economical losses. In all six cases, high tree mortality was observed in natural populations with some surviving individuals exhibiting varying levels of genetic resistance. 

Budde KB, Rostgaard Nielsen L, Ravn HP, Kjaer ED. 2016. The Natural Evolutionary Potential of Tree Populations to Cope with Newly Introduced Pests and Pathogens—Lessons Learned From Forest Health Catastrophes in Recent Decades. Current Forestry reports 2: 18-29. http://link.springer.com/article/10.1007/s40725-016-0029-9